Multirotation type encoder

ABSTRACT

A multirotation type encoder of the invention includes a first encoder  20  attached to a rotating shaft  11  of a rotating machine  10  for detecting an absolute value rotational angle within one rotation, and a second encoder for counting a multirotation amount of the rotating shaft  11  by using a speed reducing mechanism  30  by magnetic coupling and the second encoder  40  is constituted by a first magnetic gear  31  directly connected to the rotating shaft and magnetized in multiple poles and at least one second magnetic gear  32  arranged to be opposed thereto in noncontact and magnetized in multiple poles by a number of magnetic poles larger than that of the first magnetic gear  31  and is constituted to count a multirotation amount by detecting a rotational angle of the second magnetic gear by the second encoder.

TECHNICAL FIELD

The present invention relates to a batteryless multirotation typeencoder for detecting the amount of multirotation of a servomotor usedfor a robot, a machine tool or the like by an absolute value angle.

RELATED ART

There is a multirotation type encoder of a related art as shown by FIG.12. FIG. 12 is a perspective view showing the multirotation type encoderof the related art. In the drawing, numeral 10 designates a rotatingmachine, numeral 20 designates a first encoder and numeral 40 designatesa second encoder. A rotating shaft 12 of the first encoder 20 isattached with a driving gear 34 a and the gear 34 a is coupled to adriven gear 34 b. The gear 34 b is coupled to the second encoder 40 viaa rotating shaft 13.

According to the multirotation type encoder constituted in this way,when the rotating machine 10 is rotated, the second encoder 40 istransmitted with a rotational number reduced in accordance with areduction ratio determined by a ratio of teeth numbers of the gear 34 aand the gear 34 b. That is, a rotational angle of the rotating machine10 is detected by using the first encoder 20 and a multirotation amountthereof is detected by using the second encoder 40.

However, since the related art is provided with a mechanicalmultirotation transmitting mechanism, in order to increase the reductionratio, the diameter of the driven gear needs to be significantlyincreased relative to the diameter of the driving gear and there poses aproblem of bringing about large-sized formation of the apparatus, andeven when a set of gears having small reduction ratios is constituted inmultistage, there poses a problem that the mechanism is complicated andlarge-sized formation of the apparatus is brought about. Further, inbringing the gears in mesh with each other, there poses a problem thatan error is produced at a rotation detector by backlash or wear ofmeshed faces or a problem that reliability is deteriorated. Furthermore,since the rotational angle is detected by counting teeth numbers of thegears at respective stages and therefore, there poses a problem that anelectronic part and a battery are needed for storing a count number andcost and time are needed for interchanging the battery periodically.Further, in order to accurately determine a boundary reaching tworotations from one rotation, an absolute value encoder within onerotation needs to separately provide on the rotating shaft.

DISCLOSURE OF THE INVENTION

Hence, in view of the above-described drawback of the related art, anobject of the present invention is to provide a multirotation typeencoder which is small-sized even in the case in which the multirotationtransmitting portion is at a high reduction ratio, having no mechanicalcontact portion other than a bearing, having high reliability and longlife and dispensing with interchange of a battery.

In order to resolve the above-described problem, the present inventionis constructed by the following constitution.

(1) In a multirotation type encoder comprising a first encoder attachedto a rotating shaft of a rotating machine for detecting an absolutevalue rotational angle within one rotation, and a second encoder forcounting a multirotation amount of the rotating shaft by using a speedreducing mechanism by magnetic coupling, the second encoder comprises afirst magnetic gear directly connected to the rotating shaft andmagnetized in multiple poles, at least one second magnetic gear arrangedto be opposed to the first magnetic gear in noncontact and magnetized inmultiple poles by a number of magnetic poles larger than a number ofmagnetic poles of the first magnetic gear, and first magnetic fielddetecting means for detecting a rotational angle of the second magneticgear.

According to the constitution, by rotating the rotating machine, evenwhen a magnetic pole pitch of the first magnetic gear differs from amagnetic pole pitch of the second magnetic gear, rotation of the firstmagnetic gear is reduced in speed in accordance with the number of polesof the second magnetic gear via magnetic coupling and is accuratelytransmitted as rotation of the second magnetic gear. Therefore, areduction ratio can be increased by increasing the number of poles ofthe second magnetic gear and therefore, it is not necessary to increasea diameter of the second magnetic gear and even when the reduction ratiois increased, large-sized formation of the apparatus can be prevented.Further, since a mechanical contact portion is not present and rotationcan be transmitted in noncontact and therefore, a multirotation typeencoder having high reliability and long life and dispensing with abattery can be realized.

(2) The second magnetic gear is arranged to the first magnetic gear viaan air gap therebetween in a radial direction thereof.

According to the constitution, an absolute value angle of the secondmagnetic gear is detected and therefore, high accuracy is constitutedand also a battery for holding memory of angle information is dispensedwith.

(3) The second magnetic gear is arranged in an axial direction of thefirst magnetic gear to overlap the first magnetic gear.

According to the constitution, the magnetic gears are arranged tooverlap in the axial direction and therefore, small-sized formation inthe axial direction can be constituted.

(4) The second encoder comprises a third magnetic gear provided at asecond rotating shaft for supporting the second magnetic gear andmagnetized in multiple poles, and second magnetic field detecting meansarranged by at least two pieces thereof to one piece of the thirdmagnetic gear via an air gap between the third magnetic gear and thesecond magnetic field detecting means.

According to the constitution, since the third magnetic gear and thesecond magnetic field detecting means are provided, a small-sized secondencoder can be constituted and an angle of the second magnetic gear canbe detected with high accuracy.

(5) The second encoder comprises a magnetic yoke in a ring-like shapeprovided at an inner periphery of the second magnetic gear, acylindrical magnet provided at an inner periphery of the magnetic yokeand magnetized in two poles, and third magnetic field detecting meansarranged at an inner portion of the cylindrical magnet by at least twopieces thereof.

According to the constitution, small-sized formation in the axialdirection can be constituted, magnetic circuits of the speed reducingmechanism and the angle detecting mechanism portion can be isolated fromeach other by using the magnetic yoke and therefore, magneticinterference with each other is not present, smooth speed reduction canbe carried out and accuracy of detecting the rotational angle is alsopromoted.

(6) The first encoder comprises the first magnetic gear and fourthmagnetic field detecting means arranged to the first magnetic gear viaan air gap therebetween at least by two pieces thereof.

According to the constitution, the first magnetic gear serves tofunction as the speed reducing mechanism and function as a constituentelement of the magnetic type encoder for generating a magnetic field andtherefore, small-sized formation can be constituted.

(7) The first magnetic gear is formed in a cylindrical shape andmagnetized to generate a magnetic field in one direction uniformly in adirection orthogonal to a center axis thereof.

According to the constitution, the magnetic type encoder is constitutedby providing the fourth magnetic field detecting means at the innerportion of the first magnetic gear and therefore, further small-sizedformation can be constituted.

(8) A magnetic damper comprising a magnetic body is arranged at an outerperipheral portion of the second magnetic gear via an air gaptherebetween.

According to the constitution, outside rotational vibration can beabsorbed and therefore, a highly reliable multirotation type encoderagainst external vibration can be provided.

(9) The magnetic damper is arranged on an extended line connecting thecenter of the first magnetic gear and the center of the second magneticgear.

According to the constitution, the magnetic damper is arranged at aposition at which the magnetic damper is operated in a direction opposedto the magnetic attractive force generated between the first magneticgear and the second magnetic gear, thereby, sizes of bearings derivedfrom the magnetic attracting force generated between the first magneticgear and the second magnetic gear can be reduced, small-sized formationcan be constituted and long life formation can be achieved.

(10) The magnetic damper is arranged at an inner peripheral portion of aframe substantially in a cylindrical shape for supporting the rotatingshaft and shafts of the magnetic gears by a structure integratedtherewith.

According to the constitution, the respective magnetic gears aremagnetically shielded and therefore, a multirotation type encoder strongat an external magnetic field can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a multirotation type encoder showing thefirst embodiment of the invention.

FIG. 2 is a schematic view showing the direction of magnetizing amagnetic gear in FIG. 1.

FIG. 3 is a waveform diagram showing a result of measurement of themultirotation type encoder according to the first embodiment.

FIG. 4 is a perspective view of a magnetic gear showing a secondembodiment of the invention.

FIG. 5 is a perspective view of a multirotation type encoder showing athird embodiment of the invention.

FIG. 6 is a perspective view of a multirotation type encoder showing afourth embodiment of the invention.

FIG. 7 is a schematic view showing a constitution of a second magneticgear according to the fourth embodiment of the invention.

FIG. 8 is a perspective view of a multirotation type encoder showing afifth embodiment of the invention.

FIG. 9 is a perspective view of a multirotation type encoder showing asixth embodiment of the invention.

FIG. 10 is a plane view of a magnetic gear portion of a multirotationtype encoder showing a seventh embodiment of the invention.

FIG. 11 is a sectional view of a magnetic gear portion of amultirotation type encoder showing an eighth embodiment of theinvention.

FIG. 12 is a perspective view showing a multirotation type encoder of arelated art.

BEST MODE FOR CARRYING OUT THE INVENTION

An explanation will be given of embodiments of the invention inreference to the drawings as follows.

FIRST EMBODIMENT

FIG. 1 shows a first embodiment of the invention. FIG. 1 is aperspective view of a multirotation type encoder showing the firstembodiment of the invention. In the drawing, numeral 20 designates afirst encoder for detecting an absolute value angle, numeral 30designates a speed reducing mechanism comprising a first magnetic gear31 and a second magnetic gear 32, numeral 40 designates a secondencoder, and numeral 41 designates first magnetic field detecting means.FIG. 2 illustrates schematic views of the magnetic gears of the speedreducing mechanism 30. FIG. 2( a) shows the first magnetic gear 31 ofFIG. 1 and FIG. 2( b) shows the second magnetic gear 32 thereof. Anarrow mark in the drawings indicates a magnetizing direction. Meanwhile,the second magnetic gear 32 is magnetized in a circumferential directionat a number of portions thereof.

The first magnetic gear 31 having a diameter of 5 mm is magnetized in 2poles in a direction orthogonal to a rotating shaft. The diameter of thesecond magnetic gear 32 is set to 5 mm, the number of poles thereof isset to 16 poles and the clearance between the magnetic gears is set to0.3 mm. Further, the second magnetic gear 32 is arranged at thesurrounding of the first magnetic gear 31 via an air gap therebetween ina radial direction. Therefore, when portions of the first magnetic gear31 and the second magnetic gear 32 having different polarities arefacing to each other, an attracting force is operated therebetween andby constituting a transmitting force thereby, rotation of the firstmagnetic gear 31 is transmitted to the second magnetic gear 32.

The second encoder 40 comprises the second magnetic gear 32 and thefirst magnetic field detecting means 41 and detects the amount ofmultirotation amount of a rotating shaft 11 by detecting the rotationalangle of the second magnetic gear 32.

Next, operation of the encoder having the constitution will bedescribed. When the first magnetic gear 31 is rotated, rotation of thefirst magnetic gear 31 is reduced in speed and transmitted to the secondmagnetic gear 32. A reduction ratio at this occasion is prescribed by aratio of magnetic pole numbers of the first magnetic gear 31 and thesecond magnetic gear 32. Here, ratios of magnetic pole numbers of secondmagnetic gears 32 a, 32 b, and 32 c relative to that of the firstmagnetic gear 31 are respectively designated by notations L, M, N. Whenby using second encoders 40 a, 40 b, and 40 c, a rotational angle of thesecond magnetic gear 32 a by a unit of 1/L circumference, a rotationalangle of the second magnetic gear 32 b by a unit of 1/M circumference,and a rotational angle of the second magnetic gear 32 c by a unit of 1/Ncircumference are detected, three kinds of signals from gears 32 a, 32 band 32 c can be provided from respective detectors. Therefore, a leastcommon multiple of L, M, N constitutes a number of combination thereofand a multirotation amount in correspondence with the least commonmultiple can be counted. For example, when L, M, N are set as L=21,M=22, N=23, a multirotation amount of 10626 times can be detected.

Further, although according to the embodiment, a number of pieces of thesecond magnetic gears 32 is set to 3 pieces, the multirotation amountcan be detected when the number of pieces is equal to or larger than 1piece.

In such the constitution, actually, the first magnetic gear 31 and thesecond magnetic gear 32 a are rotatably supported by bearings, notillustrated, the first magnetic gear 31 is rotated from outside and itis measured whether the second magnetic gear 32 is reduced in speed. Thefirst magnetic gear is rotated at 6000 rpm and behaviors of rotations ofthe first magnetic gear 31 and the second magnetic gear 32 are compared.

FIG. 3 shows a result of measurement. FIG. 3 illustrates waveforms forcomparing rotational numbers of the first magnetic gear 31 and thesecond magnetic gear 32 during a constant period of time. It isconfirmed that whereas the first magnetic gear 31 is rotated by 8rotations, the second magnetic gear 32 is rotated by 1 rotation under acondition of a reduction ratio of 1:8. That is, it is known thatalthough a magnetic pole pitch of the first magnetic gear 31 is 7.85 mmand a magnetic pole pitch of the second magnetic gear 32 is 0.98 mm andboth thereof differ from each other by a multiplication factor of about8, speed is accurately reduced and transmitted.

SECOND EMBODIMENT

FIG. 4 shows a second embodiment of the invention. FIG. 4 is aperspective view of a magnetic coupling portion comprising magneticgears showing a second embodiment of the invention.

A second magnetic gear 32 is arranged relative to a first magnetic gear31 to overlap each other in an axial direction via an air gap. The firstmagnetic gear 31 is magnetized in a longitudinal direction of a rotatingshaft and a number of poles thereof is 2 poles. Also the second magneticgear 32 is magnetized in a longitudinal direction of a rotating shaftand multiple poles are magnetized in a circumferential direction.Therefore, when portions of the first magnetic gear 31 and the secondmagnetic gear 32 having different polarities are opposed to each other,an attracting force is operated therebetween, a transmitting force isconstituted thereby, and rotation of the first magnetic gear 31 istransmitted to the second magnetic gear 32. Although operation andeffect thereof are similar to those of the first embodiment, small-sizedformation can be constituted with regard to a diameter direction sincethe first and the second magnetic gears overlap each other.

THIRD EMBODIMENT

FIG. 5 shows a third embodiment of the invention. FIG. 5 is aperspective view of a multirotation type encoder showing the thirdembodiment of the invention. In the drawing, notations 33, 33 a, 33 b,33 c designate third magnetic gears, notations 42, 42 a, 42 b, 42 cdesignate second magnetic field detecting means, and notations 50, 50 a,50 b, 50 c designate second rotating shafts for connecting the secondmagnetic gear 32 and the third magnetic gear 33. All of the magneticgears are rotatably supported by bearings, not illustrated, via thesecond rotating shaft 50. The third magnetic gear 33 is magnetizedorthogonally to the rotating shaft 50 and in one direction. Further, 2pieces or more of the second magnetic field detecting means 42 arearranged at a surrounding of the third magnetic gear 33.

Next, an explanation will be given of a method of detecting a rotationalangle of the second magnetic gear 32. When the third magnetic gear 33 isrotated simultaneously with the second magnetic gear 32, a sine wave of1 period per 1 rotation can be provided from the second magnetic fielddetecting means 42 arranged at the surrounding of the third magneticgear 33. Outputs having shapes of a sine wave and a cosine wave can beprovided from 2 pieces of the second magnetic field detecting means 42as voltage values. When respective output voltages at a certain timepoint are designated by notations Va, Vb, a rotational angle θ of thesecond magnetic gear 32 can be calculated by the following equation.θ=arc tan(Va/Vb)

Operation and effect thereof are similar to those of the firstembodiment. Further, by using a magnetic type encoder, the embodimentachieves an effect of dispensing with a battery or the like for backupwhich has been needed in an optical type encoder or the like for holdingan absolute value and reducing a number of parts and content ofmaintenance.

FOURTH EMBODIMENT

FIG. 6 shows a fourth embodiment of the invention and FIG. 7 showsdetails of the second magnetic gear 32. In the drawings, numeral 35designates a magnetic yoke, numeral 36 designates a cylindrical magnetand numeral 43 designates third magnetic field detecting means.

Although the embodiment is substantially the same as the thirdembodiment, a point of the embodiment which differs therefrom resides inthat the second encoder 40 is constituted by the magnetic yoke 35provided at an inner periphery of the second magnetic gear 32, thecylindrical magnet 36 arranged at an inner portion thereof and the thirdmagnetic field detecting means 43 arranged at an inner portion thereof.The speed reducing mechanism portion is constituted by the firstmagnetic gear 31 magnetized in 2 poles and the second magnetic gear 32similar, to the third embodiment. Further, a rotational angle detectingmechanism portion of the second magnetic gear is constituted by themagnetic yoke 35, the cylindrical magnet 36 provided on an inner sidethereof and magnetized in 2 poles in a direction orthogonal to arotating shaft and the second magnetic field detecting means 42 arrangedat a space portion on an inner side thereof. The magnetic yoke 35isolates a magnetic circuit of the magnetic speed reducing mechanismconstituted by multiple pole magnetizing magnets of the second magneticgear 32 and the magnetic circuit of the cylindrical magnet 36 therebyprevent magnetic interference from being brought about between the twomagnetic circuits.

Although operation and effect of the embodiment are similar to those ofthe third embodiment, a thin type speed reducing mechanismsimultaneously having a speed reducing function and a rotational angledetecting function can be constituted, small-sized formation in an axialdirection can be constituted and an ultra small-sized multirotation typeencoder can be realized. Further, since the magnetic interference is notpresent, smooth speed reduction of the magnetic gear can be carried outand accuracy of detecting the rotational angle can further be promoted.

FIFTH EMBODIMENT

FIG. 8 shows a fifth embodiment of the invention. In the drawing,notations 44, 44 a, 44 b designate fourth magnetic field detectingmeans. Although the embodiment is substantially the same as the firstembodiment, a point of the embodiment which differs therefrom resides inthat the first encoder 20 is constituted by arranging the first magneticgear 31 and the fourth magnetic field detecting means 44 a, 44 b at thesurrounding of the first magnetic gear 31 via an air gap therebetween.Since a sine wave and a cosine wave can be detected from the fourthmagnetic field detecting means 44, the rotational angle of the firstmagnetic gear can be known by calculating inverse tangent similar to thethird embodiment. Although operation and effect of the embodiment issimilar to those of the first embodiment, further small-sized formationcan be constituted with regard to the axial direction.

SIXTH EMBODIMENT

FIG. 9 shows a sixth embodiment of the invention. The embodiment issubstantially the same as the fifth embodiment. A point of theembodiment which differs therefrom resides in that the first magneticgear 31 is formed in a cylindrical shape, a uniform magnetic field inone direction is generated in a direction orthogonal to a center axisthereof and fourth magnetic field detecting means 44, 44 c, 44 d arearranged at inside of an air gap of the first magnetic gear 31 by apredetermined phase difference therebetween. A sine wave and a cosinewave can be detected from the fourth magnetic field detecting means 44and therefore, the rotational angle of the first magnetic gear can beknown by calculating inverse tangent similar to the third embodiment.Operation and effect thereof are similar to those of the fifthembodiment.

SEVENTH EMBODIMENT

FIG. 10 shows a magnetic gear portion which is a seventh embodiment ofthe invention. In the drawing, numeral 60 designates a magnetic damperand numeral 70 designates a frame.

According to the embodiment, the magnetic damper 60 constituted by amagnetic body is arranged at an outer peripheral portion of the secondmagnetic gear 32 via an air gap therebetween and the magnetic damper isarranged on an extended line connecting a center of the first magneticgear 31 and a center of the second magnetic gear 32.

When the second magnetic gear 32 is rotated, an eddy current isgenerated at the magnetic damper 60 constituted by the magnetic body andvibration energy is converted into thermal energy in the magnetic body.That is, the second magnetic gear 32 is operated not to rotate by beingapplied with viscous braking. Therefore, a magnetic attractive forcebetween the second magnetic gear 32 and the magnetic damper 60 isdirected in a direction opposed to the magnetic attractive force betweenthe first magnetic gear 31 and the second magnetic gear 32, as a result,a radial load supplied to bearings (not illustrated) of the firstmagnetic gear 31 and the second magnetic gear is reduced.

By providing the magnetic damper 60, rotational vibration applied fromoutside to the second magnetic gear 32 by way of the rotating shaft 11is absorbed and therefore, the vibration is not transmitted thereto.That is, a highly reliable multirotation type encoder against externalvibration can be provided.

Further, although the first magnetic gear 31 and the second magneticgear 32 are opposed to each other via a small clearance (0.1 mm orsmaller), the radial load is applied to the bearings holding therespective magnetic gears by the magnetic attractive force between thefirst magnetic gear 31 and the second magnetic gear 32, since themagnetic damper 60 is provided, it is not necessary to enlarge thebearings by an amount in correspondence with an amount of the radialload and the radial load is alleviated. Therefore, with the presentconstitution, small side of the encoder can be ensured and long lifeformation of the bearings can be achieved.

EIGHTH EMBODIMENT

FIG. 11 shows a magnetic gear portion which is an eighth embodiment ofthe invention. FIG. 11 shows a sectional view at a position the same asthat of an A-O-B section in FIG. 10 of the seventh embodiment. In thedrawing, numeral 80 designates a frame serving also as damper 80 andnumeral 90 designates a bearing.

According to the embodiment, the frame serving also as damper 80 isconstituted by integrating the frame 70 and the magnetic damper 60 ofthe seventh embodiment.

By providing the frame serving also as magnetic damper 80, the number ofused parts can be reduced, further, magnetic shielding can beconstituted thereby and therefore, influence of external magnetism canalso be prevented.

INDUSTRIAL APPLICABILITY

According to the invention, a first magnetic gear connected directly toa rotating shaft and magnetized in multiple poles and at least one pieceof second magnetic gear arranged opposedly to the first magnetic gear innoncontact and magnetized with multiple poles by a number lager thanthat of the first magnetic gear, a multirotation amount is counted bydetecting the rotational angle of the second magnetic gear andtherefore, an effect of providing a multirotation type encoder isachieved which is small-sized even in the case in which a multirotationtransmitting portion is constituted by a high reduction ratio, having nomechanical contact portion other than a bearing, having high reliabilityand long life and dispensing with interchange of a battery.

Further, a reliable multirotation type encoder can be provided againstexternal vibration by arranging a magnetic damper and small-sizedformation and long life formation thereof can be achieved.

1. A multirotation type encoder comprising: a first encoder attached toa rotating shaft of a rotating machine for detecting an absoluterotational angle within one rotation; and a second encoder for countinga multirotation amount of the rotating shaft by using a speed reducingmechanism by magnetic coupling, wherein the second encoder comprises: afirst magnetic gear directly connected to the rotating shaft andmagnetized in multiple poles; at least one second magnetic gear arrangedopposedly to the first magnetic gear in noncontact and having magneticpoles by a number larger than a number of magnetic poles of the firstmagnetic gear; and first magnetic field detecting means for detecting arotational angle of the second magnetic gear.
 2. The multirotation typeencoder according to claim 1, wherein the second magnetic gear isarranged to the first magnetic gear via an air gap therebetween in aradial direction thereof.
 3. The multirotation type encoder according toclaim 1, wherein the second magnetic gear is arranged in an axialdirection of the first magnetic gear to overlap the first magnetic gear.4. The multirotation type encoder according to claim 1, wherein thesecond encoder further comprises: a third magnetic gear, provided at asecond rotating shaft supporting the second magnetic gear, andmagnetized in multiple poles; and second magnetic field detecting meansarranged with at least two pieces thereof to one piece of the thirdmagnetic gear via an air gap between the third magnetic gear and thesecond magnetic field detecting means.
 5. The multirotation type encoderaccording to claim 1, wherein the second encoder further comprises: amagnetic yoke in a ring-like shape provided at an inner periphery of thesecond magnetic gear; a cylindrical magnet provided at an innerperiphery of the magnetic yoke and magnetized in two poles; and thirdmagnetic field detecting means arranged at an inner portion of thecylindrical magnet by at least two pieces thereof.
 6. The multirotationtype encoder according to claim 1, wherein the first encoder comprises:the first magnetic gear; and fourth magnetic field detecting meansarranged by at least two pieces thereof via an air gap between the firstmagnetic gear and the fourth magnetic field detecting means.
 7. Themultirotation type encoder according to claim 1, wherein the firstmagnetic gear is formed in a cylindrical shape and magnetized togenerate a magnetic field in one direction uniformly in a directionorthogonal to a center axis thereof.
 8. The multirotation type encoderaccording to claim 1, wherein a magnetic damper comprising a magneticbody is arranged at an outer peripheral portion of the second magneticgear via an air gap therebetween.
 9. The multirotation type encoderaccording to claim 8, wherein the magnetic damper is arranged on anextended line connecting a center of the first magnetic gear and acenter of the second magnetic gear.
 10. The multirotation type encoderaccording to claim 8, wherein the magnetic damper is arranged at aninner peripheral portion of a frame substantially in a cylindrical shapefor supporting the rotation shaft and a shaft of the magnetic gear by astructure integrated therewith.